Document DE 198 32 228 C2 discloses an antenna glazing for automobiles possessing these characteristics, in which an electrically conducting cladding is used as antenna element. To decouple antenna signals originating from the cladding placed inside the glazing made in the guise of composite glazing, a coupling electrode is coupled in the capacitive regime to said cladding. Said electrode is composed of a plurality of thin wires placed parallel to and some distance from one another, which are linked electrically at one end by means of a busbar or similar, and extend over the cladding starting from this connection. The dielectric intermediate layer of the capacitor thus formed is constituted by at least one adhesive layer of the composite.
This form of construction of a coupling electrode offers, by comparison with prior embodiments comprising a narrow contact band applied by screen-printing or a construction in the form of a sheet to one of the glazings of the composite, the big advantage that it is practically not seen in the fitted state. Almost inevitably, these coupling electrodes are in fact situated in the visual field of the glazing in question, which is installed, for example in the guise of windscreen, on an automobile with metal bodywork, since in such cases of use, it is necessary to part or cut the cladding, the whole way along the edge of the glazing, in order to avoid direct coupling of the antenna field with the bodywork (earthed). It is therefore not possible to provide said coupling electrode very near to the edge of said glazing where it might be optically hidden by the framework of the window.
If wires of, for example, 10 to 100 μm in diameter are placed inside a composite glazing or on its surface, they cannot be discerned as such under normal conditions, unless they are placed very short distances apart. If on the other hand their mutual separation is at least very nearly ten times their diameter, they may not be visible as such unless one looks at them from very near the glass glazing. With the known antenna glazing, it has been demonstrated that, compared with a plane coupling electrode, consisting for example of a band of metal sheet, the coupling capacity is, in the case of the embodiment in accordance with the invention, amply sufficient for the transmission of the signal of the antenna.
Practical applications of the thin wires coupling electrode show, however, that with the wires connected at one end only, the desired transmission power is not always obtained. For optimal optical inlay, the number of wires and hence the total width of the electrode are kept as small as possible. Within the framework of industrial manufacture, the establishment of electrical contact of the whole set of wires with the common connection electrode (at the base of the antenna) may not, however, be fully achieved. In the final analysis, there is no reliable test procedure making it possible to test the operating capacitance of the coupling electrode before and/or after the manufacture of the composite forming the glazing. If manufacture has terminated, and if defective coupling is found thereafter, the whole pane has to be scrapped.
One could contemplate lengthening the wires and allowing them to overhang on both sides, beyond the glazing, so that then a continuity check is possible. It would however still always be necessary to test each wire individually or its contact with the base of the antenna, so much so that a noticeable reduction in expenditure cannot be expected from this variant.
Document DE 42 37 818 A1 describes an antenna glazing for automobiles, on the surface of which is placed an antenna for radio signals in the form of a loop, made by screen-printing. Starting from a plane connection zone made in the zone of the edge of the glazing, a strand of the loop penetrates the visual field of the glazing as far as the inversion point, from which the other strand returns. The free end thereof forming the base of the antenna is surrounded, with a slot-shaped gap, by the plane starting zone of the first strand. This structure forms the antenna proper and is not provided for capacitive coupling with a plane antenna structure.